1. Field of the Invention
The present invention relates to an echo canceling apparatus built into sound communication apparatus capable of hands-free communications in a video conference system, telephone set using the same, and echo canceling method.
2. Description of the Related Art
In general, a hand-free communications, reception sound is input to a microphone directly or as reflected on a wall, which generates an acoustic echo. The acoustic echo itself degrades sound quality and a great degree of acoustic echo causes howling. For the IP telephone that has been recently wide spread, a delay caused by a codec and transmission is great so that the acoustic echo is easily perceived by the talker, thereby further degrading the sound quality.
This type of sound communication apparatus generally uses an acoustic echo canceller (echo canceling apparatus). The acoustic echo canceller uses an adaptive filter to estimate the characteristics of an acoustic echo path and forms a pseudo-echo signal having the same characteristics as the acoustic echo path. The acoustic echo canceller subtracts the pseudo-echo signal from the transmission sound signal thus canceling the acoustic echo component from contained in the transmission sound signal.
Adaptive learning of the acoustic echo canceller is influenced by the nonlinear characteristics of the echo signal, sound of the near end talker (at the microphone) or noise. It is thus difficult to generate a precise pseudo-echo signal. As a result, an eco signal may remain in a calculator that subtracts a pseudo-echo signal from a transmission sound signal or an error in the pseudo-echo signal is output as a residual echo, which emphasizes to the echo or causes howling in some cases.
In order to eliminate such a trouble, the ITU-T recommendation G.16 defines the configuration of the “echo suppressor” that attenuates a residual echo signal linearly or non-linearly in the post-stage of the echo canceller to improve the sound quality.
JP-A-09-116471 as a patent example of an echo suppressor describes assumption of an echo cancellation volume by comparing signals levels at the input/output of the calculator of a pseudo-echo and control of the attenuation volume in the echo suppressor. JP-T-2002-501366 describes suppression of distortion of transmission speed by placing the echo canceller in an inactive state and adjustment of the attenuation ratio of the echo suppressor that is based on the sound-to-noise ratio.
FIG. 15 is a block diagram showing a related art echo canceling apparatus.
In FIG. 15, a numeral 1 represents a microphone for sound input/output, 2 an A/D converter (ADC) for converting an analog sound signal (microphone-input sound signal) to a digital sound signal, 3 an adaptive FIR filter for generating a pseudo-echo signal from a reception signal, 4 a calculator for subtracting the pseudo-echo signal from the microphone-input sound signal, 5 a line interface circuit for communicating signals to/from a communication network 6, 7 a D/A converter (DAC) for converting digital sound data to analog sound data, 8 a speaker for outputting sound, 9 an echo cancellation volume detector for calculating the amplitude ratio of a signal before and after the calculator 4, 10 an attenuator for attenuating a transmission signal, and 11 a transmission/reception sound detector for detecting transmission and reception of sound.
The sound signal from the distant party input from the communication network 6 is converted to a linear digital sound signal via the line interface circuit 5 and then input. The sound signal branches to a path a and a path b and are input to the D/A converter 7 and the adaptive FIR filter 3. The signal transmitted to the D/A converter 7 is converted to an analog sound signal and output as sound from the speaker 8. Part of this signal becomes an acoustic echo which is input to the microphone 1 and converted to a digital sound signal by the A/D converter 2. From the reception signal passing through the path b, a pseudo-echo signal similar to the echo signal is generated by the adaptive FIR filter 3. The pseudo-echo signal generated by the adaptive FIR filter 3 is subtracted from the digital sound signal output from the A/D converter 2 by the calculator 4 and the resulting differential signal is input to the attenuator 10.
The echo cancellation volume detector 9 calculates the echo cancellation volume based on the calculation of an amplitude ratio of the input signal to the calculator 4 to the output signal that has undergone correction of delay by the calculator 4. The amplitude ratio of sound from the transmitter (at the speaker 8) is close to 1 so that the echo cancellation volume can be calculated only when the reception signal is detected by the transmission/reception sound detector 11. The attenuator 10 changes the gain in accordance with the result of transmission/reception sound detection although the range of gain variation is determined in accordance with the result of the echo cancellation volume detector 9. A simple configuration (a transmission/reception sound switch) is possible where the range of gain variation of the attenuator 10 is fixed without the echo cancellation volume being detected and attenuation is made only in accordance with detection of transmission/reception sound.
As described in JP-T-2002-501366, a more complicated configuration is also used where the attenuator is controlled based on the transmission/reception sound detection or sound-to-noise ratio.
In order efficiently suppress a residual echo in the attenuator 10 of reception sound in related art echo canceling apparatus, it is necessary to correctly estimate the echo cancellation volume and the absolute volume of the residual echo signal contained in the output signal of the calculator 4, and adjust the suppression volume accordingly. In case the estimated value is smaller than the actual value, the suppression volume in the attenuator 4 is insufficient to cause the residual echo to be perceived. In case the estimated value is greater than the actual value, the suppression volume in the attenuator 4 becomes excessive and the residual echo as well as the transmission sound is suppressed, which will cause an unfamiliar sound quality.
The related art echo canceling apparatus has been accompanied by a problem that it is difficult to precisely estimate the absolute volume of a residual echo signal. This problem is due to the fact that the beginning and end of each transmission/reception sound sequence is difficult to detect so that it is difficult to clearly discriminate the residual echo of transmission sound from that of reception sound. Moreover, the environmental noise of the talker is output from the calculator. The resulting error will increase depending on the environment of the talker.
In this way, according to the related art echo canceling apparatus, estimation of the residual echo volume resistant to an error in transmission/reception sound detection or to environmental noise is difficult. A method for estimating the residual echo volume is not described in JP-A-09-116471 or JP-T-2002-501366. As a result, attenuation of the residual echo is insufficient or transmission sound is attenuated thus causing unfamiliar sound quality.
The echo canceling apparatus, the telephone set using the same and the echo canceling method does not require detection of transmission/reception sound but requires resistance to environmental noise.